It is known that a carbon fiber-reinforced resin composition has conventionally been used as an alternative for a metal or a glass-fiber reinforced composition. For example, they have been used as automobile components, household equipment components or the like. However, as compared with a metal or a glass fiber-reinforced composition, a carbon fiber-reinforced resin composition has poor strength, and hence, the range in which it is used as an alternative was limited. The reason therefor is that, since a polyolefin resin is non-polar resin, interface adhesion of carbon fibers is poor. Therefore, carbon fibers cannot exhibit effects of improving the strength as a reinforcing agent sufficiently.
As a method for improving the interface adhesion of polyolefin resins and carbon fibers, a method in which an acid-modified polyolefin resin is added to a matrix resin, a method in which carbon fibers are subjected to a sizing treatment by using a sizing agent formed of a polyolefin resin and a silane coupling agent, and, as disclosed in Patent Documents 1 to 3, a method in which carbon fibers are subjected to a sizing treatment by a sizing agent which comprises acid-modified polypropylene as an essential component are known.
However, in the method in which an acid-modified polyolefin resin is added to a matrix resin, a large amount of an acid-modified polyolefin resin is required to be added. Therefore, it can not necessarily be a good method in respect of recycling property and economy. In a method in which a sizing treatment is conducted by a sizing agent containing a silane coupling agent, effects of improving interface adhesion are not significant since carbon fibers contain not so many hydroxyl groups on the surface as compared with glass fibers. Further, a method in which a sizing treatment is conducted by a sizing agent containing acid-modified polypropylene as essential components attains a relatively good interface adhesion to glass fibers. However, in the case of carbon fibers, such effects are not sufficient.